The present invention relates to radio communications, and more particularly to radio communications methods in a cellular or similar wireless communication system between a base station transmitter/receiver (transceiver) and a plurality of remote user terminals, in particular for radio communication in a changing environment.
In such communication systems it is desirable to use directional antenna systems such as smart antenna systems to increase the signal-to-noise ratio of the communications link and reduce interference. The use of smart antenna systems can also provide resistance to multipath and fading.
A smart antenna system includes an array of antenna elements and a mechanism to determine the smart antenna processing strategy to increase the signal-to-noise ratio and/or reduce interference. A smart antenna system may be a xe2x80x9cswitched beamxe2x80x9d system that includes a beamformer forming several fixed beams and a mechanism for combining one or more of the beams. A smart antenna system may alternately be an adaptive antenna array system that includes a smart antenna processing strategy determining mechanism that can achieve an infinitely variable antenna radiation pattern that can be adapted according to the processing strategy for the particular receiving or transmitting situation.
Smart antenna systems may be used for communication on the uplink (from a user terminal to a base station) or on the downlink (from a base station to a user terminal) or on both phases of communication.
Smart antenna systems may also permit spatial division multiple access (xe2x80x9cSDMAxe2x80x9d). With SDMA, more than one user terminal of a base station may communicate with the base station on the same xe2x80x9cconventionalxe2x80x9d channel, that is, the same frequency and time channel (for an FDMA and TDMA system) or code channel (for a CDMA system), so long as the co-channel user terminals are spatially separated. In such a case, the smart antenna system provides for more than one xe2x80x9cspatial channelxe2x80x9d within the same conventional channel.
The transmission RF and interference environments can be relatively rapidly changing in a cellular system. In a packetized system, these environments may significantly change between sequential packet transmissions. Consider, for example, a cellular system that includes a base station that has a smart antenna system and one or more remote user terminals. In a rapidly changing environment, the determining of the appropriate smart antenna processing strategy needs to be adaptive to an uplink signal received from the mobile user during a time interval closely corresponding to the transmission period. Such adaption typically uses a radio signal from the user terminal to the base station, with the smart antenna processing strategy determined using such a received signal.
There is a need in the art for adapting to a rapidly changing RF and interference environment.
Polling
Consider a cellular system that includes several base stations, each having a set of one or more user terminals. It is known in the art how to determine the smart antenna processing strategy for a smart antenna system of a particular base station to achieve interference mitigation from co-channel user terminals that may be transmitting signals in the same channel but to other base stations. Such interference mitigation may be achieved by receiving radio signals at the particular base station from the interfering co-channel user terminals and distinguishing the desired signal from the interfering signals.
The particular base station may not be able to mitigate interference from other base stations"" user terminals on the uplink, or mitigate towards other base stations"" user terminals on the downlink. The particular base station may not have an adequate radio-frequency link to the other user terminals or may not have information on how to poll the other base stations"" user terminals.
Initiating Communication
When initiating communication with a remote user terminal, the remote user terminal may be logged off the system or may be in an xe2x80x9cidlexe2x80x9d state in which no communication is taking place or has taken place relatively recently between the base station and the user terminal, or in which communication takes place at a relatively slow rate with substantial silent periods.
Initiating communication between a base station and a user terminal that may be in an idle state can be relatively difficult. The location of a user terminal may be unknown because, for example, it is mobile. Furthermore, interference patterns may be rapidly varying, so that even if the location is known, there may be considerable interference present that may reduce the likelihood of successful reception of the initiating (e.g., paging) message by the base station. Furthermore, the channel for paging may be heavily used by user terminals of other base stations. In such cases, the interference to the desired/intended user terminal may be considerable.
It is often desirable to page the user terminal on a conventional channel that may be heavily used on different spatial channels by other remote terminals of the same base station. In such a case, the interference to the user terminal may also be considerable.
Sending a paging message to page a user terminal is typically ideally carried in some manner that increases the likelihood that a user terminal at an unknown and possibly changing location in an environment with rapidly varying interference will successfully receive such paging (and other control signals) from its associated base station.
One embodiment is a method that includes sending a first poll from a first base station having a smart antenna system to a first user terminal prior to transmitting downlink data to the first user terminal. The first base station receives a first uplink response signal from the first user terminal as a result of the first user terminal successfully receiving the first poll. The first base station also receives one or more other uplink response signals from one or more other remote user terminals as a result of at least one other remote user terminal successfully receiving a second poll from a second base station. The first base station transmits downlink data to the first user terminal using a downlink smart antenna processing strategy for transmitting to the first user terminal, including mitigating interference to the first and one or more other remote communication devices from which the first base station received a first or other uplink response signal and that may be receiving during transmission of downlink data to the first user terminal. The downlink smart antenna processing strategy is using the first and other received uplink response signals. The first base station uses a first protocol with its associated user terminals and the second base stations uses a second protocol that is coordinated with the first protocol to enable the first base station to receive the first and other uplink response signals from second and one or more other remote communication devices that may be receiving during the step of transmitting downlink data to the first user terminal.
One embodiment also is a method of communicating a downlink data from a first base station that has a smart antenna system to a first user terminal. The method includes providing a first set of sequential time intervals for the first base station. Each of the time intervals has a selected number of downlink channels and an associated channel on the uplink for each of the downlink channels. The associated uplink channel is for the first base station to receive an uplink response signal from the first user terminal in response to the first user terminal""s receiving a signal from the first base station on the downlink channel associated with the associated uplink channel. The method further includes transmitting at least a first downlink polling signal from the first base station to the first user terminal on a first downlink channel of a first time interval of the first set, receiving a first uplink response signal at the first base station on the associated uplink channel associated with the first downlink channel of the first set in response to the first downlink polling signal, receiving at least one other uplink response signal at the first base station on a second associated uplink channel associated with a second downlink channel of a second set of sequential time intervals provided for a second base station. Each of the time intervals of the second set has a selected number of downlink channels, and an associated channel on the uplink for each of the downlink channels, and each second set of sequential time intervals is coordinated with the first set of sequential time intervals. The other uplink response signal is from at least one other remote user terminal in response to data transmitted from the second base station on a second downlink channel of a time interval of the second set of sequential time intervals. The method further includes determining a downlink smart antenna processing strategy for transmitting from the first base station to the first user terminal using the received first and other uplink response signals. The determined downlink strategy includes interference mitigation towards the other remote communication device. The method further includes transmitting downlink data from the first base station to the first user terminal using the determined downlink smart antenna strategy on a downlink channel of the first set associated with the associated uplink channel on which the first uplink response signal is received. The coordination is such that the associated uplink channels of the first and second set enable the first base station to receive the first and other uplink response signals received at the first base station and to distinguish the first uplink response signal from the other uplink response signal.